The parts,

then, in animals that are not homogeneous with
themselves and uniform in their texture, both parts external and parts
internal, have the properties above assigned to them.

2

In sanguineous animals the homogeneous or uniform part most
universally found is the blood, and its habitat the vein; next in
degree of universality, their analogues, lymph and fibre, and, that
which chiefly constitutes the frame of animals, flesh and whatsoever
in the several parts is analogous to flesh; then bone, and parts
that are analogous to bone, as fish-bone and gristle; and then, again,
skin, membrane, sinew, hair, nails, and whatever corresponds to these;
and, furthermore, fat, suet, and the excretions: and the excretions
are dung, phlegm, yellow bile, and black bile.

Now, as the nature of blood and the nature of the veins have
all the appearance of being primitive, we must discuss their
properties first of all, and all the more as some previous writers
have treated them very unsatisfactorily. And the cause of the
ignorance thus manifested is the extreme difficulty experienced in the
way of observation. For in the dead bodies of animals the nature of
the chief veins is undiscoverable, owing to the fact that they
collapse at once when the blood leaves them; for the blood pours out
of them in a stream, like liquid out of a vessel, since there is no
blood separately situated by itself, except a little in the heart, but
it is all lodged in the veins. In living animals it is impossible to
inspect these parts, for of their very nature they are situated inside
the body and out of sight. For this reason anatomists who have carried
on their investigations on dead bodies in the dissecting room have
failed to discover the chief roots of the veins, while those who
have narrowly inspected bodies of living men reduced to extreme
attenuation have arrived at conclusions regarding the origin of the
veins from the manifestations visible externally. Of these
investigators, Syennesis, the physician of Cyprus, writes as follows:-